Cannot post record out of range

We are using Paradox 11 on a purchase order system.  We use a form to
create
the POs.  The top of the form updates a PO master file, and the bottom
updates PO details, with the items to be purchased.  

Recently, we have been receiving the message "Cannot post record out of
range"  whenever we move from the top of the form (PO master file) to the
bottom (PO details).  This happens when we try to add a new PO.  I have
searched the tables and the form and cannot find a range definition
anywhere.

When I check the PO master table, the PO record is not posted.  I can
force
the post to happen by trying to insert a new record.  After that, we can
call
the PO up and fill in the detail records.

This message even happens when I delete POs that were created before the
problem started and add them again (testing to see what range limits there
may be).  It also happened after I deleted all records in a test copy of
the
files, so it does not appear to be a problem with files too full.  I have
also rebuilt both files to ensure that the indexes were OK.

Any help would be appreciated.

BHeavilin